The present invention relates to an automatic exposure device for a photocopying machine and more particularly to an automatic exposure device which automatically causes the output light intensity of the copying machine light source to be changed in accordance with the density of the original document so as to produce copies of the document of proper density.
There have conventionally been provided automatic exposure devices of the pre-scanning type and of the real-time type.
An automatic exposure device of the pre-scanning type is designed to cause the optimum power to be applied to a light source to enable copying of the lowest density portion of the original document, after the original is once exposed and before performing the image forming operation.
An automatic exposure device of the real-time type is designed to set up an optimum power to be applied to the light sorce with respect to the density of an original at each point of sensing the density of the original while the image forming operation is being performed.
In either of above systems, it is necessary to accomplish the initial setting of the relationship regarding the received light intensity and the proper light intensity (hereinafter, this relationship is referred to as "light exposure characteristic") so as to perform automatic exposure of light, and the initial setting of the above-mentioned relationship is accomlished manually.
To describe the above further in detail, the density of a copy is visually confirmed during the manual operation by copying a newspaper and a standard original, and adjustments are made to manually set the initial setting is manually of the light exposure characteristic based on the density of the copy obtained.
To make adjustments in the above, copying of the newspaper and standard original is repeated a number of times, causing problems in which not only the general work becomes complicated to accomplish the initial setting of the light exposure characteristic but also the time required for the work is extended.
An automatic exposure device of the pre-scanning type requires exposing of an original prior to performing the image forming operation, and so requires a long time before the first image of each original is formed. On the other hand, in an automatic exposure device of the real-time type, because the power to be applied to the light source is to changed in accordance with the density of the original at each point of exposure, there will be a problem in which, if there exists a black or solid area on the original, an area on both sides of the solid area will remain white wherein no image is formed, making it impossible to accomplish normal image formation which corresponds to character information and the like within that area.